This invention is directed to an isostatic pressurizing bag, a method of making this bag and a method of using the bag in consolidating and/debulking fibers and resins into composite structural parts.
Because of their high strength coupled with light weight, composite parts are being increasingly used as structural components in a variety of articles. Typical composite part materials include glass or graphite fibers that are embedded in resins such as epoxy, phenolic or bismaleimide resins. Generally the fiber and resin is "laid up" over a die or a mold and then cured under elevated temperature and pressure. Composite parts destined to be used in the aerospace or aircraft industries must meet exacting requirements as to their structural integrity. To meet these exacting requirements, precise control of temperature and pressure conditions is required during their cure.
A widely utilized system for forming structure composite parts utilizes what are termed "prepregs". These are sheets of fiber that have uncured resin embedded therein. The prepreg is laid over a die or mold and while being maintained in position against the die or mold it is subjected to heat and pressure to cure the prepreg into the composite material. Various apparatus are utilized to maintain the prepreg in contact with the die or mold during heat and pressure treatment. Such apparatus includes press platens, vacuum bags or trapped rubber molding systems.
Pressurizing the composite part during curing by the use of press platens is limited to essentially only planar parts due to the constrictions of geometry. Vacuum bagging is a much more versatile technique and has been used extensively. In vacuum bagging a prepreg is laid against a die or a mold. Depending upon the prepreg the prepreg may be overlaid with an appropriate bleed cloth or barrier cloth. The mold, prepreg and any bleed cloth or barrier cloth are covered with a vacuum bag. Vacuum is applied to the inside of the vacuum bag to remove all air and other volatiles between the vacuum bag and the mold surface. The mold, prepreg and vacuum bag assembly is then loaded in an oven or autoclave. Curing is accomplished by simultaneously heating and pressurizing the part in the oven or autoclave.
While vacuum bagging is very useful, in molding complicated parts vacuum bags have a tendency to bridge over concave areas and apply uneven pressure. Because of this it is very difficult to cure such complicated parts. As a result there is a high rejection rate. Thus, while vacuum bagging has many desirable features, it also has restrictions and limitations especially when it is used for complicated parts.
In trapped rubber molding a closed container is utilized. A portion of the interior of the container includes a mold or die surface that defines the part surface. A prepreg is located on such mold or die surface followed by an appropriate bleed cloth and/or barrier cloth. The remainder of the interior of the container is then filled with a preshaped solid silicone rubber member. This member is chosen to have a high coefficient of thermal expansion. The container is closed and heated. This causes the silicone rubber member to expand and in doing so it consolidates or debulks the prepreg against the mold or die surface. When the apparatus is properly configured, it applies pressure to the prepreg lay up on the die.
Fixed volume type trapped rubber molding systems are extremely simple from a mechanical standpoint since they have no moving parts. However, it is very difficult to control the pressures generated in these systems. Pressures can exceed desired and safe limits. If the solid silicone rubber member in a fixed volume container has been cured at or near room temperature and the prepreg and rubber member essentially fill up all of the void volume of the mold when it is assembled, upon heating it is very difficult to control pressures within the mold. Molds have been known to catastrophically fail due to excess pressure. To circumvent such control difficulty elaborate systems have been devised such as that described in U.S. Pat. No. 4,889,668. In this patent a trapped rubber mold system is described that utilizes a variety of pressure and temperature sensors as well as heating and cooling coils that are located in different areas of the mold. Such complications inevitably lead to increased costs of the molded part.
In order to circumvent the generation of excess pressure it has been proposed to form the solid elastomer of a trapped mold system of such a size that it does not fill the totality of the interior of the mold. In this type of trapped rubber molding the lay up, i.e. the prepreg, is not pressurized against the mold surface until a specific temperature has been reached. While this procedure does result in controllable pressure in certain uses, it does not permit maintaining pressure at the beginning or at the end of the cure cycle. Further, such a system is not necessarily uniform in the pressures achieved during continuous cycles. What was a proper ullage for the first cycle will not necessarily be the proper ullage for the second cycle, third cycle or the like. A given ullage for a desired pressure-temperature relationship during a first cycle may not necessarily be repeatable for other cycles thereafter.
In U.S. Pat. No. 4,148,597 assigned to the same assignee as this application, hydraulic pressurized inflatable diaphragms were utilized to exert pressure on a prepreg against a mold or die surface. As an extension of such fluid pressurization, recently certain silicone rubbers have become available that normally exist in a pulverized powdered state at room temperature but are converted to a gel when subjected to pressure. In U.S. Pat. Nos. 4,704,240 and 4,755,341 use of such pulverized rubber is described.
In U.S. Pat. No. 4,704,240 a prepreg is positioned on the inside of a tubular type mold to mimic the tubular shape of the mold. An expandable bag is then positioned in the interior of the tubular shape of the prepreg. The remaining space within the tubular shaped prepreg is filled with the particulate rubber. The mold is closed and heated. Pressure is applied to the prepreg by inflating the bag in the interior of the mold. The inflation pressure of the bag is transferred to the pulverized rubber and from there to the prepreg to consolidate the prepreg against the tubular mold surfaces.
In U.S. Pat. No. 4,755,314 a prepreg is positioned on a mold and covered with pulverized rubber. The prepreg, mold and pulverized rubber are then covered with a vacuum bag. External pressure applied to the vacuum bag is transferred to the pulverized rubber and from there to the prepreg surface.
One inherent disadvantage of utilizing the pulverized rubber system described in the above referenced U.S. Pat. Nos. 4,704,240 or 4,755,341 is the degradation of the pulverized rubber resulting from elevated temperatures and/or pressures. The pulverized rubber tends to degrade or decompose during use with the by-products of its decomposition or degradation contaminating the surface of the composite part that is being formed. Since these by-products generally are oily silicone type by-products, they are very difficult to remove from a composite part surface and can interfere in bonding of the composite part to an adjacent composite part during final assembly of a multiple composite article.
To circumvent the degradation of a composite part surface by the decomposition products of particulated silicone rubbers, U.S. Pat. No. 4,795,600 suggests encapsulating the composite prepreg with a coating that is capable of being separated from the composite part after cure of the prepreg into a composite part. The coatings have to be chemically removed from the composite part after formation of the composite part. The preferred coating suggested by the patent is aluminum foil. It is removed in an alkali etch. Other suggested coatings are polymer films that are removed utilizing solvent etches. The use of such films is labor intensive Further having to subject the finished composite part to either an alkali or chemical etch subjects the composite part to possible degradation and/or contamination by the etch.